This invention relates to spacecraft attitude and stationkeeping control systems by the use of throttleable or pulse-modulated thrusters, and particularly to such systems in which unified thruster logic allows use of most of the remaining thrusters in the presence of a failure of a single one.
Spacecraft are widely used for communications, earth sensing and exploration, vehicle locating, and for surveillance. All of these uses require that the orientation of the spacecraft in space, and possibly its location or station, be accurately controlled. The spacecraft attitude may be controlled by magnetic torquers, by momentum or reaction wheels, or by the use of thrusters.
Since each spacecraft at launch has at least a slightly different configuration than other spacecraft, its thrusters will be placed at slightly different locations relative to the center of gravity than in other spacecraft. Also, the specific impulse characteristics of the thrusters may differ from one to the next. As propellant or other consumables are expended, the center of mass or gravity of the spacecraft may move. All of these effects contribute to variability in torque imparted by the various thrusters. As a consequence, the logic for controlling the thruster system is ordinarily custom-written for each spacecraft. During the construction phase for a spacecraft, it may be found to be necessary to alter the thruster configuration or characteristics, which may require costly rewriting of software.
Control arrangements ordinarily provide torques by preselected combinations of thrusters. The selected thrusters are ordinarily located on opposite sides of the center of gravity. When in orbit, a spacecraft thruster may fail or may have a significant change in specific impulse characteristics. When a thruster fails, those combinations of torque for which the failed thruster is an element cannot be used. This may adversely affect or limit control of the spacecraft.
On a geosynchronous satellite, the North (or South), East and West faces are ordinarily fitted with thruster arrays. For North-South (N-S) stationkeeping, North-face (or South-face) thrusters provide acceleration, and for East-West (E-W) stationkeeping, the East and West face thrusters provide acceleration. Attitude control is provided by operating various combinations of N, E and W thrusters to achieve the desired 3-axis torque. Prior art thruster control is ordinarily selected to provide redundancy in case of failure of a single thruster or of an entire half-system (odd or even). A half-system failure could result from failure of an element common to half the thrusters, as for example failure of a common manifold valve. Prior art controllers divide the operation into a plurality of "modes", each of which is treated separately and each of which is associated with a distinct, independent logic, in either hardware or software form. For reliability purposes, the thrusters are redundant, with odd and even thrusters connected to different propellant tanks.
In case of a failure of one of the thrusters, the corresponding half-system is ignored because of the complexity of the logic required to use the remaining operable thrusters. Use of only half the thruster system results in reduction in performance and reliability, additional use of fuel and reduced spacecraft lifetime.
A unified thruster control arrangement is described in copending U.S. patent application Ser. No. 07/552,638, filed Jul. 18, 1990 in the name of Paluszek et al. As described therein, spacecraft control is achieved by sensing the spacecraft attitude, and generating torque and force vectors T and F, respectively, which represent the forces and torques which the thrusters are to produce. Each of the torque and force vectors includes three mutually orthogonal components T.sub.1, T.sub.2 and T.sub.3, and F.sub.1, F.sub.2 and F.sub.3, respectively. Control is achieved by forming a plurality of difference equations having the form: ##EQU1## where a and b are the maximum torque and force, respectively, which the j.sup.th thruster can produce, and o is the throttle setting of the j.sup.th thruster, which may take on values ranging from zero to unity. The difference signals .DELTA. are summed to form a single scalar equation relating variable .alpha. to a scalar performance index Z. The single scalar equation is solved for that value of .alpha..sub.j which maximizes Z, and the thrusters are controlled in a manner directly related to the corresponding value of .alpha..sub.j. This scheme is effective, but computationally intensive, and the flight computer must therefore have sufficient power to perform the abovementioned computations on an ongoing basis, together with other ordinary spacecraft control activities. The computational requirements result in a slowing of the control response time, or impose a requirement for a high-speed or more complex computer.
A unified thruster control system is desired which reduces on-board computer computation.